So does that mean that if you wanted to port, say, Xmonad to Wayland, you would have to reimplement compositing logic again? (Since the existing compositing-logic implementation also handles things like window placement that you would want to override)

In theory, yes. In practice, you could build something on top of wlroots, which does a lot of the work for you. In fact, check this out:

https://github.com/swaywm/hsroots

Thank you for the clarifications :)

An X window manager would likely have to be completely rewritten to work on Wayland, I doubt many of them have much abstraction of the underlying window system.